Electronics assembly applications such as integrated circuits and silicon-based sensors generally include multiple layers of metal traces that terminate in metal bond pads through which electrical signals are transmitted. These bond pads need to be protected from environmental conditions such as high humidity, which are known to degrade the performance of an electronics assembly.
Bond pads are often made of aluminum because aluminum or gold bond wires are readily attached to the bond pads, yet the aluminum bond pads are susceptible to corrosion under standard environmental test conditions. This corrosion can cause performance degradation and product failure when the joint between a gold wire and an aluminum pad degrades and fails.
Semiconductor manufacturers have begun replacing aluminum bond pads on integrated circuits and sensors with copper bond pads, which have superior electromigration performance as well as lower resistivity. Copper is an attractive alternative to aluminum if manufacturing processes can avoid atmospheric contamination of the copper surface, which oxidizes to form a coating that is not readily removable by standard methods of wirebonding machines, and usually requires flux for soldering interconnections. Processing approaches have been developed for copper metallurgy to control or limit oxidation that tends to reduce the conductance of the copper bond pads.
A proposed solution for protecting metal bond pads of conventionally packaged, non-hermetic chip-on-board assemblies is to encapsulate the bonded die with a silicone compound, which helps isolate the pads from aggressive environmental conditions such as high humidity. Unfortunately, dispensing and curing the silicone is a time-consuming process. The silicone, having a higher dielectric constant and loss tangent than air, may cause a degradation of high-frequency and radio-frequency (RF) performance. In addition, silicone encapsulations are difficult to remove completely, precluding rework and repair.
The difficulty of protecting bond pads from environmental attack is acknowledged by Polak and others in “Protecting Electronic Components in Acidic and Basic Environment”, U.S. Pat. No. 6,030,684 issued Feb. 29, 2000. In the proposed process, electronic components are encapsulated in a modified fluorosilicone with an acid-base buffer dispersed within the polymeric material. Unfortunately, fluorosilicones can degrade RF performance of RF devices operating at high frequencies and can be difficult to remove.
Inorganic protective thin films such as silicon nitride or oxide that have been used to protect microsensor structures are disclosed in “Media Compatible Microsensor Structure and Methods of Manufacturing and Using the Same”, Maudie et al., U.S. Pat. No. 5,889,211 issued Mar. 30, 1999. The microsensor structure includes a microsensor package, a microsensor device, a leadframe, a connective wire, a leadframe, and an inorganic protective film formed on all or a portion of the exposed surfaces of the structure. The film or coating, which is vacuum-deposited, cannot be used with chip-on-board (COB) applications and is an expensive process, particularly when applied at the assembled device level.
A solution with selectively encapsulated bond pads has been proposed in “Micro Electro-Mechanical System Sensor with Selective Encapsulation and Method Therefor”, Monk et al., U.S. Pat. No. 6,401,545 issued Jun. 11, 2002. Monk and others use selective encapsulation in which a polymeric or wafer-bonded silicon dam is used to prevent the flow of encapsulant onto a micromachined pressure sensor diaphragm, while allowing the encapsulant to still protect the wirebonds and pads. This approach does not address the RF performance and repairability problems for COB applications. The selective encapsulation of the microelectromechanical system (MEMS) sensor protects wirebonds, while permitting the pressure sensor diaphragm to be exposed to ambient pressure without encumbrance or obstruction.
Petrovic and others describe protecting a MEMS pressure sensor with a hydrophobic and oleophobic polytetrafluoroethylene filter, alone or in combination with silicone encapsulation in “Physical Sensor Component”, European Patent Application EP 1,096,243 published May 2, 2001 and U.S. Patent Application US2002/0050170 published May 2, 2002. The housing of the physical sensor component has a cavity with a pressure sensor device mounted inside, and a chemically selective and physically selective filter overlying the cavity and separated from the pressure sensor device. While this approach is effective, it is not compatible with chip-on-board assembly applications and adds bulk.
A two-component encapsulation method that allows rework of an electronic module or removal of integrated circuits is described by Warren in “Top of Die Chip-on-Board Encapsulation”, U.S. Pat. No. 5,951,813 issued Sep. 14, 1999. A first encapsulant is applied only to the bonds and pads on the die and a second more easily removed encapsulant is applied to the wire bond spans and wire bonds on the substrate. This more complex process is incompatible with MEMS sensing requirements and does not address the RF performance degradation problem for COB applications.
The copper surface of copper bond pads are generally not suitable as a terminal metal for packaging interconnections, and as a result, some manufacturers have coated the pads with other deposited metals such as palladium and nickel to reduce or eliminate voids in the copper or at the interface between the copper and the nickel. The copper bond pads may be activated with a thin layer of palladium in a palladium activation bath to allow deposition of a nickel layer thereon. One exemplary approach for coating copper bond pads is disclosed in “Method for Processing a Semiconductor Substrate Having a Copper Surface Disposed Thereon and Structure Formed”, Molla et al., U.S. Pat. No. 6,362,089 issued Mar. 26, 2002. A dual activation process is used to produce a semiconductor wafer having metal-coated copper bond pads. The bond pads are activated in a palladium bath, placed in a nickel-boron bath, and then coated with a layer of nickel-phosphorous or palladium. In this process, the nickel-phosphorous or palladium layer is optionally coated with a layer of gold for subsequent formation of solder balls or wirebonds thereon.
It would be beneficial to provide an improved method for passivating and protecting wire-bondable copper bond pads of integrated circuits, sensors and chip-on-board assemblies from corrosion without requiring an encapsulation material such as silicone. In addition, such a method would result in circuits, sensors and assemblies that do not require a complex cleaning process, have repairable and reworkable bond pads, have improved reliability of electrical connections to the integrated circuits, and do not have degraded RF performance. The method would accommodate full wafers or singulated die from various vendors with varied pad metallurgy. The method would accommodate analog and digital integrated circuits, memory die, RF die, sensor die, sensor assemblies, wireless assemblies, and electronic assemblies. The finish would be wire bondable and corrosion resistant. The desired approach would allow low-cost plastic packages to be used in some copper bond-pad applications that have previously required costly, hermetic ceramic packages, and would overcome the deficiencies and obstacles described above.